BOTANY. 



egenblt; bination, these juices are enabled to act variously on 

 the luminous rays. When the alkali predominates, 



"*v the more refrangible rays, as the violet, blue, and 

 green, are ivtleeted, and the other rays are extinguish- 

 ed ; when the cieid prevails, the least refrangible, or 

 red rays, are reflected, and the others disappear; and 

 from intermediate admixtures of these ingredients, in- 

 termediate colours, both simple and compounded, will 

 arise. The colours, howev*r, which these juices pre- 

 sent to our sight, are not reflected by the coloured 

 particles, but by the opaque matter on which they 

 are imposed, so that the coloured matter transmits, 

 but does not reflect light ; and this light arriving at 

 the eye, produces an impression, which conveys the 

 sensuuon of the individual colour. 



" Hence, too, it follows, that, when light is whol- 

 ly excluded, the chemical changes in the vegetable 

 juices, which enable them to exert these actions on 

 the colorific rays, do not take place, and, consequent- 

 ly, the green colour of the leaws, which depends on 

 the predominance of alkaline matter, and the red co- 

 lours of leaves and of flowers, which arise from an 

 excess of acid, are equally prevented from appearing j 

 for the juices being unable, in this state, to decom- 

 pose the solar beam, return it almost or entirely un- 

 changed to the eye, whence the objects are destitute 

 of colour, or have the appearance of whiteness. The 

 colours of plants, therefore, depend primarily on the 

 chemical action of light, in changing the constitution 

 of their juices ; and these juices, by their physical 

 operation on the colorific rays, are then enabled to 

 exhibit all thir infinite variety of hues." 



Siich is the broad outline of the facts established by 

 Mr Ellis in the course of his researches. It now re- 

 mains for us to add a few particular details, by which 

 all apparent inconsistency may be avoided. It will 

 have been remarked, that tke carbonic acd is said to 

 be formed exteriorly to the surface of the leaves. The 

 carbon employed in the earliest stages of germina- 

 tion, " we suppose," says Mr Ellis, " to be given 

 off by the operation of the same causes, and nearly 

 in the same manner, as it is afforded by inanimate 

 bodies. By this carbon the surrounding oxygen is 

 changed, and the developi-ment of the seed succeeds 

 to this chemical action! la the progress of this de- 

 developement, the organization of the seed is unfold- 

 ed ; a, id, when this is sufficiently complete, the einix 

 A ion of carbon, like the other function'; of the seed, 

 is then executed by an organised structure, and 

 becomes obedient to those laws which govern and 

 regulate the actions of living beings." The car- 

 bon of vegetables is afforded by a living aetion ; 

 and Mr Ellis maintains, with much force, that it 

 " passes off in combination with their exhaled fluids," 

 ind then combines with the oxygen of the atmo. 

 sphere. On the other hand, the decomposition of 

 the carbonic acid is a distinct operation, subser- 

 vient to other purposes, and accomplished in the pa- 

 renchymatous substance of the leaf: So that these 

 operations may he, and actually are, co-existent in the 

 same plant. All these operations are decidedly dib- 

 tinct from those that go on in dead vegetable mat- 

 ter, and are regulated by laws as distinct as life and 

 death are. Thus, besides producing considerable 

 Aariges in the circulating fluids, the carbonic acid 



VOL. IV. PARTI. 



appears to perform oilier functions of uo meau mo- 

 ment : One "f the most interesting of these, the co- 

 louration, has been already sufficiently detailed ; an- 

 other, perhaps of higher importance, the heat produ- 

 ced, now remains to be noticed. 



Animal:!, as well as plants, require a certain degree Heat, 

 of heat to maintain their existence. The former, at 

 least a considerable number of them, possess the 

 power of maintaining a certain temperature under 

 most circumstances, so as to resist the influence of 

 external cold. It now remains for us to inquire, how 

 far such a similar power is vested in vegetables ; and, 

 if it does exist, by what means its operations are con- 

 tinued. That heat is necessary for supporting the Power of 

 vegetative principle is well known, and is illustrated re-i-ting 

 by the destruction of ife in all plants that are expo- 

 sed to a more intense degree of cold than their natu- 

 ral habits can endure. It is also pretty well determi- 

 ned, that plants do possess the power of generating a 

 certain portion of heat, and of maintaining it, unless 

 novel and unnatural causet. destroy that power. The 

 heat of plants is well shewn in winter in a meadow, 

 where we always find, that the snow dissolves more 

 readily than on the adjacent bare ground. " If," says 

 Willdenow, " in a strong frost, we put vessels with 

 water close to such a tree, we shall find that the water 

 is converted into ice, but that the tree retains its sap 

 unfrozen, and remains quite unhurt. The case is 

 different in plants of warm and hot regions. The 

 sap of these plants congeals at the least degree of 

 cold, and the plants decay. Thus there appears a 

 remarkable difference between the plants of warm and 

 those of cold climates. As long as plants live, and 

 possess sufficient vital power to resist cold, their sap 

 will not congeal. But after the buds have been forced 

 out by the warm weather of spring, they will, when 

 exposed to cold evenings, be observed to congeal. 

 We find, likewise, that dead or diseased branches are 

 more liable t be frost bitten than living and sound 

 ones ; and that branches, by their sap being congeal- 

 ed, are destroyed. The birch, and some other plants, 

 it is well known, often have their roots covered with 

 ice, without suffering the least injury. In the north- 

 ern hemisphere of our globe, there are many exten- 

 sive tracts of pine trees, which resist, with their ever- 

 green branches, the most severe winter cold. From 

 these observations it follows, that each plant, accord, 

 ing to its species, possesses a peculiar degree of 

 warmth, which defends it against the inclemency of 

 the weather," 



The most remarkable example of the heat in vege- 

 tables is given by M. Hubert. He relates, that the 

 spadices of the Arum cord/folium, which grows in Ma- 

 dagascar and the Isle of France, indicated a most re- 

 markable degree of temperature, in the course of 

 so.ne observations that he made upon them. On ap- 

 plying a thermometer to five spadices which had un- 

 folded in the preceding night, a rise of 2,5 from ttie 

 temperature of the atmosphere took place ; the fol- 

 lowing morning, the difference between the tempera- 

 ture of the spadices and of the surrounding atmo- 

 sphere was reduced to 21, and in the evening of the 

 same day to 7. He also observed, that the mule 

 part* of the spadices raised the temperature of the 

 thermometer 13 higher than the female organs <Jf 

 i 



